The invention relates generally to lightwave communication systems and, more particularly, to techniques for transporting information at high speeds in dense wavelength division multiplexing systems.
Optical fiber has become the transmission medium of choice for communication networks because of the speed and bandwidth advantages associated with optical transmission. Wavelength division multiplexing (WDM), which combines many optical signals at different wavelengths for transmission in a single optical fiber, is being used to meet the increasing demands for more speed and bandwidth in optical transmission applications. With recent advances in optical networking technology, system manufacturers are now contemplating dense wavelength division multiplexing (DWDM) systems that carry, for example, as many as 40, 80, or more channels in a single fiber and with bit rates up to 10 Gbps per channel. In these DWDM systems, aggregate bandwidth in a single fiber is a function of the number of wavelength channels and the bit rate per wavelength channel.
DWDM is generally regarded as a channelized technology because a separate wavelength channel is allocated for carrying traffic from each source or user. Despite the many advantages of DWDM, the channelized nature of DWDM imposes limitations on transporting information in existing systems and networks. For example, wavelength exhaust becomes a problem because the number of users that can be supported by a DWDM system is limited by the number of available wavelengths. The total bandwidth in a DWDM system may also be used inefficiently if all wavelengths are not being used to transport information at the maximum possible bit rate. Excessive overhead, e.g., hardware and software, is yet another problem associated with the channelized nature of existing LDWDM transport schemes. In particular, because each wavelength channel is typically dedicated to carrying a specific type of traffic, each wavelength channel therefore requires its own hardware and software resources. For example, each wavelength channel may transport information at a different bit rate and therefore require a dedicated clock recovery mechanism and dedicated synchronization and recognition circuitry at the receiving end to process the serially transmitted bit stream. Consequently, cost and complexity will increase as a function of the increase in number of wavelength channels.
Transporting information in existing DWDM systems is also limited by commercially available electronic and photonic components. For example, electronic circuitry, e.g., synchronization circuitry used in receivers, typically cannot process data at the same speeds as the photonic components used for optically transmitting data in the optical fiber. Even if higher speed circuitry was available, this circuitry would add more cost and complexity to the system.
Latency or delays associated with multiple parallel-to-serial and serial-to-parallel conversions of data is another problem with existing DWDM transport schemes. For example, while most communication sources supply parallel formatted information (e.g., voice and video codecs, ASCII data generators, etc.), information is still transmitted serially in each wavelength channel of a DWDM system. As a result, undesirable delays are caused by the multiple parallel-to-serial and serial-to-parallel conversions occurring in the transmission path.
High speed transport in a dense wavelength division multiplexed system without the latency and bandwidth limitations of prior systems is achieved according to the principles of the invention by transmitting information in a parallel format using a subset of the total number of optical channels in a wavelength division multiplexed signal as a parallel bus transmission group.
Information supplied by multiple sources, which may be operating at different transmission rates, is multiplexed into a parallel format and transmitted at the same transmission rate in each of the optical channels in the parallel bus transmission group.
According to one illustrative embodiment of the invention, a selected number of optical channels in a wavelength division multiplexed signal are allocated to form a parallel bus transmission group. Because each optical channel in the parallel bus transmission group is associated with a particular wavelength, the parallel bus transmission group can also be referred to as a wavelength bus. Information from one or more sources is supplied in a parallel format 30 and then transmitted at the same transmission rate in each of the optical channels in the wavelength bus. When information from more than one source is to be transported, the information from each source is multiplexed into a parallel format. More specifically, parallel byte interleaving is used so that parallel formatted information supplied by one source is byte interleaved with parallel formatted information from another source and so on. The wavelength bus architecture is highly scalable in that a wavelength division multiplexed signal can be partitioned into multiple wavelength buses with each wavelength bus sized according to the bandwidth requirements of the traffic to be transported. In particular, the wavelength bus is sized by changing the number of optical channels, i.e., wavelength channels, by changing the transmission bit rate for the wavelength channels in the wavelength bus, or by a combination of both. Additionally, a single wavelength bus can transport a combination of differently formatted traffic (e.g., SONET, ATM, IP, etc.) as well as traffic supplied at different bit rates.
By transmitting information in parallel using a wavelength bus architecture, delays otherwise associated with parallel-to-serial and serial-to-parallel conversions are substantially reduced as compared with existing systems. Limitations associated with the channelized nature of existing DWDM systems are also overcome according to the principles of the invention. For example, problems relating to wavelength exhaust and inefficient use of bandwidth are overcome because a separate optical channel (i.e., wavelength) is not required for each user or traffic source when using the wavelength bus architecture according to the principles of the invention. Furthermore, because information is transported in parallel using multiple optical channels in a wavelength bus, fewer clock recovery mechanisms and less complex synchronization and recognition circuitry may be used as compared to existing WDM transport schemes. More specifically, existing systems require this type of circuitry on a per-channel basis (e.g., a phase locked loop for each bit stream), whereas the wavelength bus architecture facilitates the sharing of circuitry across an entire group of optical channels.